In wireless communication systems such as cellular systems, a reference signal (RS) is introduced, which is used for acquiring various indices of a propagation path and a transmission signal. For example, in LTE (Long Term Evolution) and LTE-advanced (Rel. 10) of a next-generation system, which are reviewed in the 3GPP (3rd Generation Partnership Project) that is an international standardization group for mobile communications, an SRS (Sounding Reference Signal) is used as one of reference signals used in an uplink channel. In an uplink channel of a wireless communication system, as the uplink communication, data is transmitted from a transmission apparatus (for example, a terminal (User Equipment: UE)) to a reception apparatus (for example, a base station (e-NodeB; eNB).
“Sounding” refers to an estimation of a channel quality. The SRS is time-multiplexed together with an uplink data symbol and is transmitted for mainly allowing the reception apparatus (for example, a base station) for receiving uplink data to estimate the channel quality of an uplink data channel in the propagation path.
Hereinafter, an example of a method of transmitting an SRS will be described (see Non-Patent Literature 1). In this example, a transmission apparatus (for example, a terminal) for an uplink channel transmits an SRS in a narrow band while chancing a transmission frequency band in a predetermined time, and a reception apparatus for the uplink channel performs sounding of a wide band using several narrow-band SRSs.
FIG. 1 is a diagram that illustrates an example of a method of transmitting an SRS using frequency hopping described in Non-Patent Literature 1. In each cell of the wireless communication system, as a transmission band (narrow band) for the SRS, one of four transmission bandwidths (SRS hopping BW: b=0 to 3 in FIG. 1) can be set to a terminal. In addition, as a frequency hopping range, one of the above-described SRS BWs (SRS Hopping BW=bhop=0 to 3 in the drawing) can be set.
As SRS parameters used for setting a frequency range, an initial transmission band position is set together with the SRS transmission bandwidth b and the frequency hopping range bhop described above. By substituting these SRS parameters into a predetermined equation that represents a hopping pattern (the amount of change in frequency), the SRS transmission band at each transmission timing at the time of performing frequency hopping is determined.
In addition, in a case where the SRS transmission bandwidth of a terminal is set to be equal to or larger than the frequency hopping range (in other words, b≦bhop), the frequency hopping is not applied to the terminal. As above, in a conventional method of transmitting an SRS, the frequency hopping of continuous frequency bands can be set.
On the other hand, in the next version of LTE-Advanced (Rel. 11), in order to further improve the capacity of a communication channel, a Heterogeneous Network (HetNet: Heterogeneous Network) using a plurality of base stations having mutually different coverage areas is under review. More specifically, for example, the operation of a HetNet is under review in which a pico cell (also called a Low Power Node (LPN) or a low-power Remote Radio Head (RRH)) is arranged within a coverage area of a macro cell (also called a High Power Node (HPN)).
In such a HetNet environment, the interference (uplink channel interference) from a terminal (hereinafter referred to as a macro terminal (Macro UE) that is controlled by the base station (Macro eNB) of the macro cell to a terminal (hereinafter, referred to as a pico terminal (Pico UE)) that is controlled by the base station (Pico eNB) of the pico cell becomes a problem.
FIG. 2 is a diagram that illustrates an example of uplink channel interference in the HetNet environment. It is necessary for an uplink transmission signal of the macro terminal (Macro UE) 11 to be received at an appropriate level by the base station 12 (Macro eNB) of the macro cell 10. Accordingly, as the uplink transmission power of the macro terminal 11, power that is used for compensating for a path loss between the macro terminal 11 and the base station 12 of the macro cell 10 is set. On the other hand, as the uplink transmission power of the pico terminal (Pico UE) 21, power that is used for compensating for a path loss between the pica terminal 21 and the base station (Pico eNB) 22 of the pica cell 20 is set.
Here in a case where the macro terminal 11 is located in an area located near the cell edge of the macro cell 10 (hereinafter, referred to as a cell edge area), or in a case where the macro terminal 11 is located behind an obstacle such as a building at which it is difficult to receive a direct wave from the base station 12 of the macro cell 10, or in other cases, the path loss between the macro terminal 11 and the base station 12 of the macro cell 10 is large.
At this time, it is assumed that the uplink transmission power of the macro terminal 11 is higher than the uplink transmission power of the pico terminal 21. In other words, in such circumstances, there is a possibility that an uplink transmission signal of the macro terminal 11 becomes a significant interference factor for the uplink transmission signal of the pica terminal 21. Especially, in a case where the macro terminal 11 is located near the pica cell 20, the influence of the uplink channel interference will further increase.